I would steer away from using a username as a login. A person's default username may be taken on your site, in cases like these people usually use a variation by appending some numbers/characters. Now, when they try to login they'll use the default username first because they don't remember. Using email removes all these barriers.

Why even add a username? I usually don't add them unless they are necessary (in cases where a user's profile, other information needs a pretty url)